In general, polypeptides are marginally stable in the aqueous state and undergo chemical and physical degradation resulting in a loss of biological activity during processing and storage. Another problem encountered in aqueous solution in particular is hydrolysis, such as deamidation and peptide bond cleavage. These effects represent a serious problem for therapeutically active polypeptides which are intended to be administered to humans within a defined dosage range based on biological activity.
To reduce the degradation of polypeptides, water-based pharmaceutical compositions are generally kept refrigerated or frozen until ready for use. As an alternative, the process of freeze-drying is often employed to stabilize polypeptides for long-term storage, particularly when the polypeptide is relatively unstable in liquid compositions. A lyophilization cycle is usually composed of three steps: freezing, primary drying, and secondary drying; Williams and Polli, Journal of Parenteral Science and Technology, Volume 38, Number 2, pages 48–59 (1984). In the freezing step, the solution is cooled until it is adequately frozen. Bulk water in the solution forms ice at this stage. The ice sublimes in the primary drying stage, which is conducted by reducing chamber pressure below the vapor pressure of the ice, using a vacuum. Finally, sorbed or bound water is removed at the secondary drying stage under reduced chamber pressure and an elevated shelf temperature. The process produces a material known as a lyophilized cake. Thereafter the cake can be reconstituted prior to use.
The standard reconstitution practice for lyophilized material is to add back a volume of pure water (typically equivalent to the volume removed during lyophilization), although dilute solutions of antibacterial agents are sometimes used in the production of pharmaceuticals for parenteral administration; Chen, Drug Development and Industrial Pharmacy, Volume 18, Numbers 11 and 12, pages 1311–1354 (1992).
Lyophilization is considered one of the best ways to remove excess water from polypeptide solutions. The freeze-drying process may yield products that are stable and amenable to handling for long-term storage. Lyophilized products can be stored at room temperature and are therefore easier to handle and distribute to a wider geographic market, such as foreign markets where refrigeration may not be available.
Excipients have been noted in some cases to act as stabilizers for freeze-dried products; Carpenter et al., Developments in Biological Standardization, Volume 74, pages 225–239 (1991). For example, known excipients include polyols (including mannitol, sorbitol and glycerol); sugars (including glucose and sucrose); and amino acids (including alanine, glycine and glutamic acid).
In addition, polyols and sugars are also often used to protect polypeptides from freezing and drying-induced damage and to enhance the stability during storage in the dried state. In general, sugars, in particular disaccharides, are effective in both the freeze-drying process and during storage. Other classes of molecules, including mono- and di-saccharides and polymers such as PVP, have also been reported as stabilizers of lyophilized products.